I would be concerned about doing that from a safety perspective. Even though it's only pulling 15 amps at max (((45Wx4)/12V)=15A) that's still a fair amount of extra load to put on the factory wiring in addition to whatever load would already be present on whatever line you were considering splicing into.
If you're dead set on not doing the safe thing and running a new, fused power line from the battery through the firewall, then pretty much your only option would be to look in the passenger foot well for the fuse box and tap into a fuse rated for higher than 15A. I can't remember if there are any there or not. If you go this route, use an add-a-fuse and don't just wrap the wire around a prong and shove it in.